Intruder States and the Onset of Deformation in the Neutron-Deficient Even-Even Polonium Isotopes

Alpha- and beta-decay studies of mass-separated Rn and At nuclei reveal the existence of a low-lying $0^{+}$ state in ${}_{}{}^{196,198,200,202}{}_{}{}^{}\mathrm{Po}$. The excited $0^{+}$ states are interpreted as proton-pair excitations across the $Z=82\mathrm{}$ shell gap leading to a deformed state, coexisting with the spherical ground state. It is shown that with decreasing neutron number the deformed configuration intrudes to lower excitation energy, increasingly mixing into the ground state.